What is "augmented octave"

n. (context music English) A musical interval of the Western twelve-semitone system consisting of thirteen semitone and spanning eight degrees of the diatonic scale. It is one semitone wider than a perfect octave and enharmonically equivalent to a minor ninth/compound minor second.

In modern Western tonal music theory an augmented octave is the sum of a perfect octave and an augmented unison or chromatic semitone. It is the interval between two notes, with the same note letter on staff positions an octave apart, whose alterations cause them, in ordinary equal temperament, to be thirteen semitones apart. In other words, it is a perfect octave which has been widened by a half-step, such as B and B or C and C; it is a compound augmented unison. It is the enharmonic equivalent of a minor ninth.